1. Field of the Invention
The invention relates in general to a communication apparatus and an associated estimation method.
2. Description of the Related Art
FIG. 1 is a schematic diagram illustrating a conventional technology which connects a communication apparatus and a remote apparatus through a transmission line. For example, the communication apparatus 101 can be a switch, and the remote apparatus 112 can be a personal computer. In practical application, the communication apparatus 101 and the remote apparatus 112 can transmit data via various types of mechanisms.
Each of the communication apparatus 101 and the remote apparatus 112 includes a transmitter and a receiver. The transmitting and receiving operations in the communication apparatus 101 and the remote apparatus 112 are compensate and corresponding to each other. As the transmitting and receiving operations of the communication apparatus and the remote apparatus are similar, only the operations of the communication apparatus 101 are described.
The communication apparatus 101 generates a positive and a negative transmission signal (Tx+ and Tx−) through the transmitter 102. The positive and the negative transmission signal (Tx+ and Tx−) are transmitted to the remote apparatus 112 through a connector and a transmission line. The connector 104 includes a transformer, a positive output impedance Rs+, and a negative output impedance Rs−. The positive and the negative output impedance (Rs+ Rs−) matches the impedance of the transmission line.
Similarly, the remote apparatus 112 transmits data to the communication apparatus 101 through the transmission line 111. In response to the remote apparatus 112, a receiver 103 is used by the communication apparatus 101 to receive a positive and a negative reception signal (Rx+ and Rx−).
As indicated in FIG. 1, the positive transmission signal Tx+, and signal the positive reception signal Rx+ share the same wire. The negative transmission and reception signal (Tx− and Rx−) share the same wire. If the design of the communication apparatus 101 is poor, the receiver 103 receives the positive and the negative transmission signal (Tx+ and Tx−) sent from the transmitter 102. Instead of receiving only the positive and the negative reception signal (Rx+, Rx−) transmitted from the remote apparatus 112, the signal received by the communication apparatus 101 is mixed with the positive and the negative transmission signal (Tx+, Tx−) and the positive and the negative reception signal (Rx+, Rx−).
When any of the reception signals (the positive/negative reception signals Rx+ and Rx−) received by the receiver 103 is interfered by the transmission signals (the positive/negative transmission signals Tx+ and Tx−) from the transmitter 102, such phenomenon is referred as echo phenomenon. Echo phenomenon is an important issue while manufacturing the communication apparatus 101.
As well as the echo phenomenon, the communication apparatus also needs to consider the problem of electromagnetic interference (hereinafter, EMI).
For the transmitter of FIG. 1, a loop is formed at the left hand side of the transformer. Meanwhile, a positive and a negative loading cu rents (ILp and ILn) respectively flowing off and flowing into the communication apparatus are equivalent. In such case, the impedance affect less on the loop current of FIG. 1.
Due to cost consideration, communication products with AC coupling are now available in the market. With AC coupling mechanism, cost of transformer is reduced. Although cost can be reduced by such replacement, the problem of current mismatch arises. Furthermore, current mismatch will cause EMI to the communication apparatus.
According to the specifications of network joint (RJ45), ideal impedance of the network wire is 50Ω. Before the communication apparatus leaves the factory, 50Ω is used as the positive output impedance Rs+, and the negative output impedance Rs−. However, tolerance of an RJ45 impedance is up to 15%. This implies that the impedances of the loading terminal may range between 42.5Ω to 57.5Ω. The impedances of the loading terminal being not conformed to ideal magnitudes defined in specifications will cause current mismatch in the communication apparatus.
FIG. 2 is a schematic diagram illustrating another conventional technology which connects the communication apparatus and the remote apparatus through the transmission line.
FIGS. 1 and 2 basically have the same design except that the transformer of FIG. 1 is replaced by two capacitors for AC coupling. The communication apparatus 201 generates the positive and the negative transmission signal (Tx+ and Tx−) through the transmitter 202. The positive and the negative transmission signals (Tx+ and Tx−) are transmitted to the remote apparatus 212 through the connector 204 and the transmission line 211. The receiver 203 receives the positive and the negative reception signal (Rx+ and Rx−) generated by the loading terminal 210.
In FIG. 2, a common mode current will be generated between the communication apparatus 201 and the loading terminal 210 when the impedances at the two ends of the loading terminal 210 in the communication apparatus 201 are not symmetric.
The common mode current occurs when magnitudes of the positive and the negative loading currents (ILp and ILn) are not equivalent.
In FIG. 2, the communication apparatus 201 adopts the AC coupling mechanism. Hence, a common mode current will be generated due to the asymmetric resistances of the loading terminal 210. Once the common mode current is transmitted back to ground, a current loop will be formed, and noises and EMI phenomenon will accordingly occur.
Along with the development of communication products, the requirement of EMI protection is getting more strict. Take network products for example, the requirement of EMI protection in new standard is 30 dB higher than the old one. This implies that EMI phenomenon of network products is required to be reduced to one thousandth of the old one.
To be comply with the strength standard of EMI, a metal shielding method is normally used to block EMI phenomenon according to conventional technologies. However, the metal shielding method is a passive approach, and does not actually reduce the strength of EMI phenomenon. Moreover, such approach uses extra metal, and the cost of manufacturing would increase.
As disclosed above, EMI and echo phenomenon are problems left unresolved in the conventional communication products.